The acyltrimethylsilanes
4-RC6H4C(O)SiMe3 (R =
H, Me, MeO) and β-naphthylC(O)SiMe3, upon
photolysis
in acetonitrile with 20 ns pulses of 248 nm light from an KrF* excimer
laser, give rise to the corresponding
α-siloxycarbenes ArC:OSiMe3, whose
absorption spectra (λmax between 270 and 310 nm),
lifetimes (between 130
and 260 ns), and reactivities with proton donors (ROH, mainly alcohols)
are reported. With highly acidic ROH,
such as 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP), the reaction is
of simple second order, rate constants being
in the 109 M-1 s-1 range and
virtually independent of the nature of the aromatic moiety of the
carbene. Isotopic
substitution of H in ROH by D has no effect on the rate constant for
reaction with carbene. For less acidic alcohols
such as, e.g., methanol, the reactivity of the carbenes increases with
increasing [ROH]. This behavior is interpreted
in terms of reversible adduct formation between carbene and alcohol
followed by reaction with further alcohol
molecule(s) to give product. On the basis of experiments in
the acidic and only weakly nucleophilic solvents 2,2,2-trifluorethanol (TFE) and HFIP, protonation of the carbenes leads to the
corresponding carbenium ions, whose
absorption spectra (λmax between 305 and 355 nm),
lifetimes (100 ns−5 μs in TFE), and reactivities with
nucleophiles
(halides, alcohols, and ethers) are reported. In the solvent HFIP,
the reactivities of the carbenium ions with the
alcohols and ethers increase with their concentration, in a way
analogous to that observed in the reaction of the
carbenes with the alcohols. This is explained as resulting from
reversible formation of a cation−nucleophile complex
followed by reaction of the complex with a second nucleophile molecule
which acts as a base. In solvents more
basic than HFIP, it is presumably the solvent which serves this
function.
